1. Field of the Invention
The present invention relates to a thrust control apparatus and a thrust control method for an airplane.
2. Description of the Related Art
A typical airplane engine having a variable pitch propeller employs a propeller governor that adjusts the propeller-consumed drive force by varying a pitch of the propeller and thereby controls the engine revolution speed to a set revolution speed. The engine output is controlled separately from the revolution speed by adjusting the opening of a throttle valve disposed in an engine intake passage. Therefore, an operator must simultaneously adjust the set revolution speed of the propeller governor and the set opening of the throttle valve in order to obtain a desired propeller thrust.
Japanese Patent Application Laid-Open No. HEI 8-324496 discloses an aircraft thrust control apparatus for varying both the set revolution speed of the propeller governor and the set opening of the throttle valve in an aircraft engine having a variable pitch propeller, through operation of a single lever. Since the disclosed apparatus controls the set revolution speed of the propeller governor and the opening of the throttle valve separately and simultaneously through operation of a single lever, the apparatus reduces the workload on the operator. Moreover, this apparatus varies at least one of the set revolution speed of the propeller governor and the opening of the throttle valve non-linearly in relation to the amount of operation of the single lever to facilitate conforming in advance the setting of the maneuvering characteristic to the body or airframe characteristics.
This related-art thrust control apparatus makes it possible to facilitate reduction of the workload on a pilot and conformation to the body characteristics through settings made beforehand such that the set revolution speed of the propeller governor and the opening of the throttle valve have a predetermined relationship to the amount of operation of the single lever. However, in this thrust control apparatus, since the governor set revolution speed and the throttle opening are simultaneously determined in accordance with the amount of operation of the single lever, a problem may be caused by the fixed relationship between the governor set revolution speed and the throttle opening.
More specifically, if the relationship between the governor set revolution speed and the throttle opening is fixed, the relationship between the engine revolution speed and the output torque also becomes fixed, so that an optimal engine revolution speed and an optimal output torque may not be obtained in various flight conditions. For example, at high altitudes, the pressure ejected from the engine supercharger (boost pressure) in an aircraft needs to be kept at least at a predetermined level because air ejected from the supercharger is used to pressurize the cabin. However, if the engine revolution speed decreases, the corresponding reduction in the exhaust gas flow may decrease the ejection pressure of the supercharger so that sufficient cabin pressurization may not be maintained. Therefore, a drastic reduction in the engine revolution speed at high altitudes is not desirable. In addition, under a condition that a constant throttle opening is maintained, the engine output increases as the boost pressure increases. Consequently, for example, during a descent from a high altitude where the governor set revolution speed is set relatively high in order to maintain a required supercharger ejection pressure level and, therefore, the throttle opening also becomes set to a relatively great value due to the fixed relationship between the governor set revolution speed and the throttle opening, it may become difficult or impossible to sufficiently reduce the engine output to a desired level. Thus, a fixed relationship between the governor set revolution speed and the throttle opening may cause a problem that during a descent, the propeller thrust becomes excessively great so that a sufficient descent (descending rate) of the aircraft body cannot be achieved.
Furthermore, when the boost pressure has reached an intercept point (the set maximum boost pressure) during a steady flight condition, the full load operation of the engine is normally feasible, so that it is preferable to fully open the throttle valve regardless of the engine revolution speed and thereby reduce the intake resistance. However, if the relationship between the engine revolution speed and the throttle opening is fixed, it may become impossible to fully open the throttle opening, depending on the revolution speed. Consequently, the fuel economy improvement achieved by a reduction in the intake resistance may become insufficient.
Further, if a failure occurs in a waste gate valve that controls the revolution speed of the supercharger by adjusting the exhaust gas flow into the supercharger, the boost pressure will excessively increase so that the engine output will become excessively great. In such a case, a fixed relationship between the engine revolution speed and the throttle opening makes it impossible to limit the boost pressure by adjusting the throttle opening while maintaining a constant engine revolution speed.
Moreover, a fixed relationship between the propeller revolution speed (engine revolution speed) and the throttle valve opening may make it impossible to operate the propeller or the engine at favorable efficiency points in various flight conditions.
The aforementioned problems can be reduced or solved to some extent by a conventional design that allows an operator to adjust the governor set revolution speed and the throttle opening separately. However, this conventional design cannot reduce the workload on the pilot.